[llvm-bugs] [Bug 31052] New: ThinLTO + switch table + O2 cause undefined symbols

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https://llvm.org/bugs/show_bug.cgi?id=31052

            Bug ID: 31052
           Summary: ThinLTO + switch table + O2 cause undefined symbols
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: krasin at google.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 17607
  --> https://llvm.org/bugs/attachment.cgi?id=17607&action=edit
reproducer

Consider the following program that consists of two files (the archive with the
sources is attached):

main.cc:

#include <stdio.h>

typedef int (*CallFunc) ();

CallFunc GetCallN(int i, int j);

int main(void) {
  printf("%d\n", GetCallN(1, 1)());
  printf("%d\n", GetCallN(2, 1)());
}

call.cc:

typedef int (*CallFunc) ();

static int CallX() { return 0; }
static int CallY() { return 1; }

const CallFunc Funcs0[] = {CallX, CallY, CallY, CallY};
const CallFunc Funcs1[] = {CallY, CallX, CallY, CallY};
const CallFunc Funcs2[] = {CallY, CallY, CallX, CallY};

CallFunc GetCallN(int i, int j) {
  switch (i) {
    case 0:
      return Funcs0[j];
    case 1:
      return Funcs1[j];
    case 2:
      return Funcs2[j];
    default:
      break;
  }
  return 0;
}

Let's build it with ThinLTO and LLD:
clang++ -o main main.cc call.cc -O2 -fuse-ld=lld -flto=thin

The following error will be displayed:
/usr/local/google/home/krasin/src/llvm.org/release-build/bin/ld.lld: error:
/tmp/main-fa77ab.o0 (function main): undefined symbol 'CallX()
(.llvm.5BF168B0)'
/usr/local/google/home/krasin/src/llvm.org/release-build/bin/ld.lld: error:
/tmp/main-fa77ab.o0 (function main): undefined symbol 'CallY()
(.llvm.5BF168B0)'
clang-3.9: error: linker command failed with exit code 1 (use -v to see
invocation)

- It works correctly with FullLTO:
$ clang++ -o main main.cc call.cc -O2 -fuse-ld=lld -flto && ./main 
0
1

- It works with a regular build (with lld, as well as ld).
- It works with FullLTO + Gold
- It fails with ThinLTO + Gold:
$ clang++ -o main main.cc call.cc -O2 -fuse-ld=gold -flto=thin
/tmp/lto-llvm-22f26f.o:/tmp/main-155d01.o:function main: error: undefined
reference to '_ZL5CallXv.llvm.5BF168B0'
/tmp/lto-llvm-22f26f.o:/tmp/main-155d01.o:function main: error: undefined
reference to '_ZL5CallYv.llvm.5BF168B0'
clang-3.9: error: linker command failed with exit code 1 (use -v to see
invocation)

Looking at the bitcode, -O2 causes the switch statement be converted into a
switch table:

@_ZL6Funcs0 = internal unnamed_addr constant [4 x i32 ()*] [i32 ()*
@_ZL5CallXv, i32 ()* @_ZL5CallYv, i32 ()* @_ZL5CallYv, i32 ()* @_ZL5CallYv],
align 16
@_ZL6Funcs1 = internal unnamed_addr constant [4 x i32 ()*] [i32 ()*
@_ZL5CallYv, i32 ()* @_ZL5CallXv, i32 ()* @_ZL5CallYv, i32 ()* @_ZL5CallYv],
align 16
@_ZL6Funcs2 = internal unnamed_addr constant [4 x i32 ()*] [i32 ()*
@_ZL5CallYv, i32 ()* @_ZL5CallYv, i32 ()* @_ZL5CallXv, i32 ()* @_ZL5CallYv],
align 16
@switch.table = private unnamed_addr constant [3 x [4 x i32 ()*]*] [[4 x i32
()*]* @_ZL6Funcs0, [4 x i32 ()*]* @_ZL6Funcs1, [4 x i32 ()*]* @_ZL6Funcs2]

; Function Attrs: norecurse nounwind readonly uwtable
define i32 ()* @_Z8GetCallNii(i32 %i, i32 %j) local_unnamed_addr #0 {
entry:
  %0 = icmp ult i32 %i, 3
  br i1 %0, label %switch.lookup, label %return

switch.lookup:                                    ; preds = %entry
  %1 = sext i32 %i to i64
  %switch.gep = getelementptr inbounds [3 x [4 x i32 ()*]*], [3 x [4 x i32
()*]*]* @switch.table, i64 0, i64 %1
  %switch.load = load [4 x i32 ()*]*, [4 x i32 ()*]** %switch.gep, align 8
  %idxprom5 = sext i32 %j to i64
  %arrayidx6 = getelementptr inbounds [4 x i32 ()*], [4 x i32 ()*]*
%switch.load, i64 0, i64 %idxprom5
  %2 = load i32 ()*, i32 ()** %arrayidx6, align 8, !tbaa !1
  br label %return

return:                                           ; preds = %switch.lookup,
%entry
  %retval.0 = phi i32 ()* [ null, %entry ], [ %2, %switch.lookup ]
  ret i32 ()* %retval.0
}

Whenever I change the code so that the switch table is not generated, it would
compile with ThinLTO just fine.

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